1. Field of the Invention
The present invention relates to a deflection yoke, and more particularly to a deflection yoke mounted with a leakage magnetic-field canceling coil on a screen portion thereof for effectively attenuating a leakage magnetic field produced around a cathode ray tube (CRT) as well as its front and rear portions, using a horizontal deflection coil mounted to horizontally deflect electron beams.
2. Description of the Prior Art
Generally, deflection yokes mounted on the CRTs of television receivers or monitors are largely classified into a saddle-toroidal type deflection yoke as shown in FIG. 1A and a saddle-saddle type deflection yoke as shown in FIG. 1B. Since a leakage magnetic field to be mentioned later is greater in the saddle-toroidal type deflection yoke that an in the saddle-saddle type deflection yoke, the saddle-toroidal deflection yoke will be taken to be described.
As shown in FIGS. 1A and 1B, a deflection yoke DY is constructed such that a pair of horizontal and vertical deflection coils HC and VC are mounted on the inner and outer sides of a pair of coil separators formed of a neck portion 2 and a screen portion 4.
A pair of ferrite cores 6 are provided on the inside of the wound vertical deflection coil VC as being wrapped by the vertical deflection coil VC in the saddle-toroidal type deflection yoke DY, and a pair of ferrite cores 6 are provided as wrapping the outer circumference of the vertical deflection coil VC in the saddle-saddle type deflection yoke DY. The respective pairs of ferrite cores 6 mounted in the two deflection yokes are fixed by a core clamp 8.
The deflection yoke DY is mounted in the neck portion of the CRT. When sawtooth pulses are applied to the horizontal and vertical deflection coils HC and VC, a magnetic field is generated in conformity with the Fleming's rule of left hand, so that red, green and blue electron beams emitted from an electron gun are deflected by the influence of a magnetic force, thereby forming a picture.
At this time, the magnetic field leakage outside of the CRT by current flowing through the horizontal and vertical deflection coils HC and VC of the deflection yokes DY is called as the leakage magnetic field. The horizontal and vertical deflection coils HC and VC in typical low-resolution monitors and small-sized televisions are supplied with current having a relatively low frequency, e.g., 15.75 MHz for the horizontal deflection coil and 60 Hz for the vertical deflection coil. Here, as shown in FIG. 2, leakage magnetic fields M1 and M2 are mainly generated from the horizontal deflection coil BC having the high frequency, but a frequency generated from the horizontal deflection coil HC is relatively low to result in negligible leakage magnetic field.
However, in case of the deflection yoke for high definition (HD) color monitors or large-sized televisions, current having a frequency, e.g., 35.75 MHz higher than that of the typical low-resolution monitors or small-sized televisions, is supplied to increase the leakage magnetic fields M1 and M2. The leakage magnetic field generated has a very low frequency magnetic field (VLMF) band of about 2 kHz-400 kHz, which may adversely affect a human body by causing various occupational diseases while working for a long time near the front of the color monitor, so that studies are actively carried out to attenuate the leakage magnetic field. Therefore, in order to inhibit excessive leakage magnetic current of the VLMF band beyond a predetermined quantity, a standard--an internationally adoptable security standard--stipulates below 25 nT per hour at a location spaced apart from the CRT by 5 m.
The conventional construction will be described with reference to FIGS. 3 to 6.
FIG. 3 is a front view showing one example of the deflection yoke mounted with the leakage magnetic-field canceling coil by a conventional technique. FIG. 4 is a view showing the generation of the magnetic field according to the deflection yoke shown in FIG. 3.
Referring to FIG. 3, a pair of leakage magnetic-field canceling coils 10 wound as a vertically-elongated rectangle are installed on both sides of the screen portion 4 to produce magnetic fields in the direction opposite to that of the leakage magnetic fields M1 and M2 which are formed toward the front and rear portions (Y axis) of the CRT screen caused by the horizontal deflection coil HC of the deflection yoke DY. At this time, the pair of leakage magnetic-field canceling coils 10 are vertical to the Y-axis of the deflection yoke, i.e., the front and rear portions of the screen.
The construction stated heretofore will be described in detail with reference to FIG. 4.
When the electron beams are deflected toward the right, the current applied to the horizontal deflection coil Hc generates the magnetic field M1 downwardly at the screen portion 4 of the coil separator, and the magnetic field M2 upwardly at the neck portion 2 of the coil separator. Meanwhile, an attenuation magnetic field from the leakage magnetic-field canceling coil 10 is generated in such a manner that a magnetic field M3 from the bottom side of the leakage magnetic-field canceling coil 10 is generated in the opposite direction to the leakage magnetic field M1 to counteract that from the screen portion 4 of the coil separator, but a magnetic field M4 from the upper side of the leakage magnetic-field canceling coil 10 generates in the same direction as the leakage magnetic field M2 to increase that M2 from the neck portion 2 of the coil separator.
In other words, this leakage magnetic-field canceling coil forms different magnetic fields at the upper coil and lower coil portions to allow the lower coil to decrease the leakage magnetic field on the screen portion of the coil separator, but the upper coil increases the leakage magnetic field on the neck portion of the coil separator to cause loss.
Moreover, in this manner, the number of turns of the leakage magnetic-field canceling coil must be increased to heighten the effect of attenuating the leakage magnetic field at the deflection yoke in the HD television monitor, large-sized television, or the like which is supplied with current having a high frequency to result int great leakage magnetic field.
If the number of turns of the leakage magnetic-field canceling coil is increased, inductance of a horizontal deflection circuit connected to the leakage magnetic-field canceling coil is increased to degrade sensitivity, i.e., the precise deflection of the electron beams in accordance with the current, of the deflection yoke.
Consequently, there is a limitation to increase the turns of the leakage magnetic-field canceling coil without affecting the product characteristics such as the sensitivity of the deflection yoke, so that the conventional deflection yoke has a problem of impeding effective elimination of the leakage magnetic field in the CRT.
In addition to this problem, the increased turns of the leakage magnetic-field canceling coil elongates overall length of the coil to produce a great amount of heat.
In order to solve the problems, a deflection yoke having a leakage magnetic-field canceling coil as shown in FIG. 5 has been presented in "National Technical Report (Vol. 38, No. 4)" published on 4 August, 1992.
FIG. 5A is a front view showing another example of the deflection yoke mounted with the leakage magnetic-field canceling coil according to a conventional technique, and FIG. 5B are a side view thereof. FIG. 6 shows distribution of the magnetic field from leakage magnetic-field canceling coil shown in FIG. 5, and FIGS. 7A and 7B show the magnetic field distribution of the deflection yoke mounted with the leakage magnetic-field canceling coil shown in FIG. 5.
Referring to FIG. 5, a leakage magnetic-field canceling coil 11 of a polygonal shape similar to a mountain peaked toward the electron gun with a bottom side in parallel with the screen portion 4 of the coil separator is mounted on the screen portion 4 of the coil separator formed in the deflection yoke DY.
After providing the deflection yoke mounted with the leakage magnetic-field canceling coil formed as above on the neck portion of the CRT, a constant frequency, e.g., a sawtooth pulse of 35.75 MHz and 60 Hz, is supplied to respective horizontal and vertical deflection coils. Then, a magnetic field is formed in conformity with the Fleming's rule of left hand. Therefore, the red, green and blue electron beams from the electron gun is subjected to a deflection force formed by the magnetic field to accurately scan respective pixels on the screen, thereby forming the picture.
As illustrated in FIGS. 6, 7A and 7B, among attenuation magnetic fields M5-M7 produced in the leakage magnetic-field canceling coil 11, the attenuation magnetic field M5 from the bottom side of the leakage magnetic-field canceling coil 11 shaped as the polygon similar to the peaked mountain has an excellent attenuation effect against the leakage magnetic field M1 around the front of the screen. As illustrated in FIG. 7A, the magnetic fields M6 and M7 from the upper side of the leakage magnetic-field canceling coil 11 are produced oblique to each other, enabling more effective attenuation of the leakage magnetic field M2 generated on the rear portion of the screen when compared with the leakage magnetic-field canceling coil 10 of the elongated rectangular shape shown in FIG. 3. As a result, the leakage magnetic-field canceling coil 11 having the shape similar to the peaked mountain has an advantage of more effectively attenuating the leakage magnetic field M1 than the leakage magnetic-field canceling coil 10 of the elongated rectangular shape mentioned in FIG. 3.
However, in spite of being slight, the magnetic field produced around the portion parallel with the bottom side when using the apex of the leakage magnetic-field canceling coil as a reference affect the leakage magnetic field from the rear portion of the screen. More specifically, since this magnetic field increases the leakage magnetic field on the rear portion as much as the total vector of the magnetic field generated on the upper side of the leakage magnetic-field canceling coil, the leakage magnetic field from the rear portion of the screen cannot be effectively attenuated as by the leakage magnetic-field canceling coil of the rectangular shape. Accordingly, the primary object of eliminating the leakage magnetic field cannot be fully achieved.
Also, The leakage magnetic-field canceling coil 11 having the shape similar to the peaked mountain has a relatively shortened coil as compared with that of the leakage magnetic-field canceling coil 10 having the rectangular shape shown in FIG. 3, thereby preventing the components from being overheated.
Furthermore, the leakage magnetic-field canceling coil as described above is shaped as the polygon to involve increased coil length, thereby causing a problem in preventing the heat generation when compared with that shaped as the simple triangle.